The present invention relates to a device and a method for introducing hydrogen into flat displays.
Particularly, the invention relates to a device and method for introducing hydrogen into field emission displays (generally known in the art as xe2x80x9cField Emission Displaysxe2x80x9d or FED) and liquid crystal displays wherein the orientation of the liquid crystals is controlled by means of a plasma (generally known in the art as xe2x80x9cPlasma Addressed Liquid Crystalxe2x80x9d displays or PALC), in order to maintain the hydrogen partial pressure in these devices within a desired range of values. The main use of these types of displays is replacing the traditional television screens based on the cathodic tube, which is heavier and more encumbering. Other uses, especially in the case of PALCs, are the boards for providing traffic information, in railway stations or airports.
In principle, the internal space of a FED should be kept under vacuum, and that of a PALC plasma chamber should contain only the rare gas necessary for the plasma formation, generally helium at pressures of about 50-500 mbar. However, both devices are known to work better and particularly to maintain their functional capacity for a longer time, if small quantities of hydrogen are present inside them.
As described in the articles of Spindt et al., in IEEE Transactions on Electron Devices, vol.38, n. 10 (1991), p.2355-2363, and of Mousa, in Vacuum, Vol. 45, n. 2-3 (1994) p. 235-239, in a FED hydrogen has the function of avoiding the oxidation of the metal electron-emitting microtips; the optimal hydrogen pressure is about between 10xe2x88x925 and 2xc3x9710xe2x88x921 mbar.
In PALCs, hydrogen has the function of accelerating the decay time of the helium plasma, by accelerating the return of the single spots which form the display (in the art defined xe2x80x9cpixelxe2x80x9d) from the xe2x80x9cswitched onxe2x80x9d to the xe2x80x9cswitched offxe2x80x9d condition; a high speed of this transition is necessary for the transmission of high definition television images. Patent application EP-A-816898 can be referred to for a detailed description of the mechanisms and problems of the PALC functioning; hydrogen partial pressures of about 0,1-100 mbar, and preferably between 1 and 10 mbar, are optimal for the functioning of the PALC.
The introduction of the desired hydrogen quantities in these displays can be carried out during the manufacturing, for example by filling up with hydrogen gas after evacuation of the internal space of the FED or of the plasma chamber in the case of PALCs; the filling up operation can be carried out by means of the same (generally glass) tubulation used for the evacuation, which can later be sealed by heat compression (technique known as xe2x80x9ctip-offxe2x80x9d).
However, hydrogen is consumed during the life of these displays. In particular, the hydrogen consumption rate has been observed to be noteworthy when the display is on, while it is negligible when it is off. The reason for this behavior is believed to be the hydrogen ionization, with formation of the H+ion when displays are switched on, which in the FEDs is due to the interaction with the electronic beams and in the PALCs to the plasma formation; the thus formed H+ions are accelerated by electric fields, also present with switched on displays, against internal portions thereof, mainly the metallic microtips in the FEDs or the electrodes in the PALCs, and sorbed by these portions.
Therefore, it is necessary to provide for a possibility of supplying the gas, when it is necessary, in the internal space of these displays during their life. The systems which have been devised up to now for this purpose are based on the employment of hydrogen accumulator materials, generally zirconium or titanium based alloys which can sorb and emit hydrogen according to equilibrium conditions that are characteristic for each alloy. These alloys can be xe2x80x9cchargedxe2x80x9d with hydrogen quantities up to a few percent of their weight, by heating them to temperatures between about 50 and 200xc2x0 C. with contemporaneous exposure to hydrogen at pressures between about 10xe2x88x924 and 2 bars. The charged hydrogen can be subsequently released from the alloy when this is exposed to hydrogen partial pressures lower than the equilibrium partial pressure for the specific alloy at the specific temperature. This kind of alloys charged with hydrogen can be positioned inside the display in communication with the internal space thereof and possibly they can be heated up to temperatures between about 40 and 500xc2x0 C. when the hydrogen pressure decreases under the values above indicated for FEDs and PALCs, in order to re-establish the optimal working atmosphere in the device. The employment of zirconium or titanium alloys, based on their hydrogen sorption and emission equilibrium properties, is described for example in patent applications EP-A-716772, EP-A-838832 and JP-A-10/199454, relating to FED type displays, and in patent applications EP-A-816898, EP-A-833363 and WO 98/57219, relating to PALC type displays.
The prior art systems, although in principle effective for maintaining hydrogen at the desired levels, are difficult to be put into practice, because it is difficult to define a specific alloy with hydrogen equilibrium pressures as a function of the temperature which can generate the desired hydrogen pressures inside the displays. Specifically, the main difficulty which is found with these systems is that these alloys generally have very low hydrogen equilibrium pressures around the room temperature (the working temperatures of FEDs and PALCs), so that most of the hydrogen released by heating the alloy is subsequently sorbed again by the alloy itself when cold; at temperatures around the room temperature, the alloy works as a hydrogen getter, rather than as a source thereof, since it can sorb also the hydrogen which has been intentionally supplied into the display during the manufacturing steps.
With patent application M199A 000534 the applicant intended to provide a device and method for the introduction of hydrogen into flat devices, free from the drawbacks of the above listed methods, based on the use of a portion of passage wall between a hydrogen supply and the flat display formed of a proton conductor material. The passage of hydrogen gas through said wall is controlled by means of two electrodes, the first of which is connected to the proton conductor material surface facing the reservoir inside and the second to the surface facing the display internal space, means for heating the portion made with proton conductor material being provided. The method further necessarily comprises a continuous monitoring of the hydrogen partial pressure in the display or at least the detection thereof when it drops under a predetermined critic value, to enable the application of a suitable potential difference between the two electrodes.
Object of the present invention is to provide a device and method for introducing hydrogen into flat displays which do not need pressure detectors capable of controlling a potential difference being applied, in the desired sense, on electrodes connected to the two faces of a proton conductor material, but which provide instead a self-regulated system without any external intervention.
This object is achieved according to the present invention by means of the device features set forth in claim 1 and by means if the method features set forth in claim 7.